M. N. Khlopkin

The floating zone growth and superconductive properties of La1.85Sr0.15CuO4 and Nd1.85Ce0.15CuO4 single crystals

Abstract A technology was elaborated to produce large superconducting crystals by use of a non-crucible floating zone melting method with light radiation heating under oxygen atmosphere. Large superconducting single crystals La 2− x Sr x CuO 4 and Nd 2− x Ce x CuO 4 ( x = 0.15) were obtained with diameters and lengths up to 7 mm and 30 mm respectively, and masses exceeding 5 g. Temperatures of the superconducting transition for La- and Nd-based crystals were T c = 39 and 25 K respectively, and the transition width was of the order of 1 K. Superconducting properties were studied by measuring resistance, AC-magnetic susceptibility and specific heat in magnetic fieldsi0 to 8 T. In the temeprature dependence of the specific heat was we observed the jumps corresponding to the superconducting transition. The content of the superconducting phase in the samples is estimated to be about 80–90%.

Imperfection of the Sm sublattice and valence instability in compounds based on SmB6

The magnetic susceptibility, electrical resistance, specific heat, and thermal expansion coefficient of SmB6, Sm0.8B6, and Sm1–xLaxB6 (x=0.1 and 0.2) are measured in the temperature range T=4–300 K. The dispersion curves of the acoustic phonon branches in lanthanum-doped samples are studied. A combined analysis of the results confirms the existence of an activation gap in the electron density of states in both nonstoichiometric and lanthanum-doped compounds. The anomalies in the electronic component of the thermal expansion coefficient are associated to a considerable degree with the temperature variation of the valence and, like the magnetic susceptibility, reflect features of the f-electron excitation spectrum. It is found that lanthanum doping does not lead to significant changes in the anomalies in the phonon spectrum of SmB6. It is established that the homogeneous intermediate-valent state of the samarium ion is fairly stable and is maintained when the perfection of the Sm sublattice is violated.

Heat capacity of La1−xSrxMnO3 single crystals in different magnetic states

The heat capacity of three single-crystal samples of La1−xSrxMnO3 (x=0, 0.2, and 0.3) is measured in the temperature range 4–400 K. It is found that the heat capacity undergoes abrupt changes due to the transitions from the antiferromagnetic phase to the paramagnetic phase (x=0) and from the ferromagnetic phase to the paramagnetic phase (x=0.2 and 0.3). The phonon contribution to the heat capacity and the Debye characteristic temperatures for the La0.7Sr0.3MnO3 sample are determined over a wide range of temperatures. The electronic density of states at the Fermi level is evaluated. It is demonstrated that an increase in the strontium concentration x brings about an increase in the electronic density of states at the Fermi level. The contributions of spin waves to the heat capacity and the entropy are estimated under the assumption that the phonon spectrum remains unchanged upon doping with Sr.

Angular dependence of the specific heat of La1.85Sr0.15CuO4 in superconducting mixed state

Abstract The specific heat of single crystals La 1.85 Sr 0.15 CuO 4 has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu–O plane and normal to this plane. Measurements were carried out in the temperature range 2–50 K in magnetic fields up to 8 T and with four directions of the magnetic fields: in the a – b plane (along the [100] and [010] directions) and at angles of 45° and 90° with respect to the a – b plane (along [001] and [103] directions). For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field. The dependence of the specific heat on the magnetic field H shows the feature predicted for d -wave pairing: H 1/2 T term. A fourfold symmetry characteristic of the electronic density of states in the crystalline a – b plane and a twofold symmetry in a – c plane was resolved in the magnetic field. The results show unambiguously that the in-plane and out-of-plane electronic density of states in the magnetic field is highly anisotropic and has a minimum when the field is along the a -axis and a maximum when the field makes an angle of 45° with the a and c axes. Using these results, we present an angular mapping of the electronic density of states and the upper critical field H c2 ( T ) estimated from the heat capacity measurements. These results are consistent with d x 2 − y 2 -symmetry of the bulk order parameter.

Thermodynamic and kinetic properties of an icosahedral quasicrystalline phase in the Al-Pd-Tc system

The properties of a quasicrystalline phase in the Al-Pd-Tc system are studied for the first time. X-ray investigations demonstrate that the quasicrystalline phase in the Al70Pd21Tc9 alloy has a face-centered icosahedral quasi-lattice with parameter a=6.514 Å. Annealing experiments have revealed that this icosahedral phase is thermodynamically stable. The heat capacity of an Al70Pd21Tc9 sample is measured in the temperature range 3–30 K. The electrical resistivity and magnetic susceptibility are determined in the temperature range 2–300 K. The electrical resistivity is found to be high (600 µΩ cm at room temperature), which is typical of quasicrystals. The temperature coefficient of electrical resistivity is small and positive at temperatures above 50 K and negative at temperatures below 50 K. The magnetic susceptibility has a weakly paramagnetic character. The coefficient of linear contribution to heat capacity (γ=0.24 mJ/(g-atom K2)) and the Debye characteristic temperature (Θ=410 K) are determined. The origin of the specific features in the vibrational spectrum of the quasicrystals is discussed.

Dependence of the specific heat of a La1.85Sr0.15CuO4 superconducting single crystal on the magnetic field direction in the a-b plane

The low-temperature specific heat of a La1.85Sr0.15CuO4 superconducting single crystal was investigated in magnetic fields up to 8 T and with four orientations — in the a-b plane (along the (100) and (110) directions) and at angles of 45° and 90° with respect to the a-b plane (along the (103) and (001) directions). Anisotropy was observed in the field dependence of the specific heat in the a-b plane. The specific heat was found to be minimum with the field oriented in the direction of the a axis and maximum with the field oriented in a direction making an angle of 45° with the a axis. This can be explained by the anisotropy of the energy gap, whose minimum lies along the (110) direction. For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field strength.

Specific heat of the electronic superconductor Pr1.85Ce0.15CuO4

We have investigated the specific heat of the ceramic Pr1.85Ce0.15CuO4 in the temperature range 2–800K in magnetic fields up to 8 T. We have determined the magnitude of the specific-heat discontinuity at the superconducting transition ΔC/Tc and estimate the coefficient of the electronic specific heat. In the temperature interval 5–800K we have separated out the phonon contribution to the specific heat, determined the temperature dependence of the characteristic Debye temperature Θ, and calculated the mean frequencies (moments) of the phonon spectrum. We compare the parameter values obtained in this way with data for the compound La1.85Sr0.15CuO4 having similar crystal structure, but hole conductivity. The magnitude of the specific-heat discontinuity and consequently the effective electron mass in the “electronic” superconductor Pr1.85Ce0.15CuO4 is several times smaller than in the “hole” superconductor La1.85Sr0.15CuO4. The phonon spectrum in Pr1.85Ce0.15CuO4 in the low-energy region is somewhat “more rigid,” and in the high-energy region somewhat “softer” than in La1.85Sr0.15CuO4.

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La1.85Sr0.15CuO4 are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated.

Thermoelectric power of Pr2−yCeyCuO4−ò superconductors with various content of Ce

The temperature dependence of the thermoelectric power (TEP) of superconducting ceramic Pr2−yCeyCuO4−o samples with various Ce content (0.13 < y < 0.17) has been investigated. The TEP value is small and has a weak temperature dependence within the range of 80–300 K. With increase in Ce content the TEP sign reverses from negative to positive. The comparison of the data obtained with TEP of La2−xSrxCuO4−o compound points to the significantly less value of both the effective mass of carriers and the density of their states in Pr2−yCeyCuO4−o. The Emery model for HTSC explains satisfactorily the experimental data for Pr2−yCeyCuO4−o and La2−xSrxCuO4−o and gives the correct sign and the order of TEP and effective mass value in these compounds.

Vibrational and electronic properties of the amorphous systems Ni44Nb56, Ni62Nb38, and Cu33Zr67 as derived from specific heat measurements

The specific heats of the amorphous systems Ni44Nb56, Ni62Nb38, and Cu33Zr67 were studied in the temperature range 3–273 K. The data obtained allow one to isolate the contribution due to atomic vibrations from the experimentally measured specific heat, to determine the density of electronic states at the Fermi level and the temperature dependence of the characteristic Debye parameter Θ over a broad temperature range, and to calculate a few frequency moments that characterize the vibrational spectrum. The information derived on the average characteristics of vibrational spectra is in good agreement with earlier data on inelastic neutron scattering. In transferring from Ni44Nb56 to Ni62Nb38, the density of electronic states at the Fermi level decreases and the characteristic vibrational frequencies increase. The density of electronic states at the Fermi level for Cu33Zr67 is close to that for Ni62Nb38. The characteristic frequencies of the vibrational spectrum of the Cu33Zr67 system are substantially lower (by 30%) than those of the Ni44Nb56 and Ni62Nb38 systems.